Although high-bandwidth communication channels, such as optical fiber communication channels are becoming more common, and are certainly usable for relatively straight-forward techniques for transmitting high-quality voice messages and television images, coding of such information at bit rates much lower than used in the simpler pulse-code-modulated techniques will retain significant importance for the foreseeable future. One reason is the growing need to transmit information with a high level of security over low-data-rate channels such as radio links. Another is the desire for memory-efficient systems for voice storage, voice response and voice mail. Still another reason is the desire to transmit higher-quality television images over older telephone communication links which are band-width limited, that is, they will accept only a relatively low bit rate in the digital transmission of information. For all of these purposes, vector quantization is a very powerful candidate technique.
An excellent survey of some of the problems involved in designing vector quantization coding strategies is presented in the article "Vector Quantization" by Robert M. Gray, IEEE ASSP Magazine, April 1984, at pp. 4 ff.
Nevertheless, that article does not address the problems of channel noise; and the resulting coding strategies would not be expected to be particularly robust in the presence of channel noise, that is, to be resistant to the effects of channel noise. Even though that article and the article of one of us, N. S. Jayant, "Coding Speech at Low Bit Rates," IEEE Spectrum, August 1986, pp. 58 ff, discuss some types of distortion, channel noise can present a more severe distortion problem, in that it can change one or more bits in the transmitted binary code and change the decoded signal values to values which are perceptually far removed from the intended values.
While there have been some other analyses of digital errors for relatively low-bit-rate digital coding strategies, which do not use vector quantization as we have defined it, e.g., see T. R. Crimmins et al, "Minimization of Mean Square Error for Data Transmission via Group Codes," IEEE Transactions on Information Theory, Vol. IT-15, pp. 72-78, January 1969; and N. Rydberg and C. E. Sundberg, "Analysis of Digital Errors in Nonlinear PCM Systems," IEEE Transactions on Communications, Vol. COM-24, pp. 59-65, January 1976, no such analysis has suggested a systematic way to design the final coding for vector quantization to provide resistance to the types of errors caused by noise during transmission.